The present invention addresses itself to the problem of plug reversal in a motor drive of the static controlled polyphase AC type, in particular for a drive accommodating adjustments in voltage and frequency for load regulation and control. It is not new to cause a reversal of direction of operation of a motor without stoppage. This can be done with a three-phase motor for instance by merely exchanging two phases at the stator input. However, when a motor is abruptly impelled to reverse its speed, the magnetic flux already created usually will prevent an immediate adjustment of the internal magnetic conditions to new electrical conditions imposed by the power source.
The invention proposes to supply the load with an alternating current voltage which is so produced at the instant of reversal that no such adverse effects as saturation by accumulated inductive energy, or a direct current component in the induced flux will occur.
The solution to the problem proposed by the instant invention consists in replacing at the instant of reversal the supplied AC voltage in each phase by an alternating voltage waveform of same amplitude and frequency but with a lagging phase shift of twice the time expanded into the operating quadrant at the time of reversal since the last cross-over point. When this condition is fulfilled the flux inducing voltage waveform accepts a center of symmetry on the axis at the instant of reversal which eliminates the adverse effects, and between the phases reversal takes place.
This invention is based on the observation that such lagging phase shift of twice the electrical angle since the last cross-over has the same effect as if the waveform had been given reverse polarity by a pi-phase shift and if from the operating point on such reverse waveform the progression on the trajectory were retraced back in time across the same electrical angle as reached from cross-over at the time of reversal on the direct waveform. In other words, the mirror image of a time progression is developed from the time of reversal concurrently with a change of polarity.
It is also known to generate an AC voltage wave of given amplitude and frequency with static switches so controlled as a function of time that elementary portions of the desired waveform are actually generated and combined so as to reconstruct the intended output wave. An illustration of this technique can be found in U.S. Pat. No. 3,491,282 of T. M. Heinrich et al.
It is further known to use digital counter means in order to recurrently generate a time representation of a quadrant of the intended sine wave. The accumulated count at any given time is in time relation with one operative point on the output wave within the quadrant. When the counter is allowed to count up and down throughout the half-cycle, the time representation extends from one cross-over to the next. The system otherwise recognizes synchronously the polarity changes in the outputted voltage. This invention takes advantage of the availability of these techniques, and typically those shown in patent applications Ser. No. 728,710 of A. Abbondanti (W. E. Case 46,515) filed Oct. 1, 1976 and Ser. No. 774,726 of R. Gemp (W. E. Case 46,961) filed Mar. 7, 1977.
The invention proposes at the instant of reversal to count back under digital control the time elapsed since the last cross-over. At the same time, and synchronously therewith, the waveform reconstruction is so inverted in polarity and reversed in the sequence of the reconstructing stages that the vectorial reversing effect is obtained with the outputted voltage wave supplied to the motor.
More specifically, the present invention relates to voltage controlled AC static power supplies of the harmonic neutralization type such as disclosed in U.S. Pat. No. 3,491,282 of T. M. Heinrich et al, entitled "Static Inverter Wherein A Plurality Of Square Waves Are So Summed As To Produce A Sinusoidal Output Wave". As explained in the patent, this technique consists in generating, with N inverters, a plurality of pulses of the same amplitude, at the same repetitive rate, spaced from each other between inverter stages by the same electrical angle .pi./N. The outputted pulses are amplified by N respective stage transformers having winding ratios which follow a cosine law from one to the next. The secondary windings are connected in series to sum up the dimensioned pulses, whereby a polyphase output waveform is obtained by vector addition through the transformer secondaries. The Heinrich Patent also describes how the AC output voltage of such a static generator can be controlled by varying the time of conduction of the thyristors in the inverters of each stage. Two modes of controlling the output voltage are disclosed in the Heinrich patent. One mode uses pulse width modulation; the second mode uses phase shifting. The present invention is applicable to this second mode of control of the inverters.
It is generally known to generate N square pulses staggered by .pi./N from one another with two square pulse signals one at the fundamental frequency, the other in the form of a train of pulses at 2 N times the fundamental frequency, the two signals being applied to an N-stage shift register. Such staggered set of pulses may be used in accordance with the harmonic neutralization technique of the aforementioned Heinrich Patent.
An object of the present invention is to provide a voltage-controlled static AC power supply wherein digital technique is used to generate a pulse width controlled signal for the purpose of reconstructing at a given instant a fundamental sinusoidal wave of controlled voltage which is shifted by a given phase angle from a first reconstructed fundamental sinusoidal wave.
Another object of the present invention is to provide a novel and unique type of static and digitally controlled AC motor drive capable of plug reversal by digital control.
Still another object of the present invention is by static switches under DC power to reconstruct a symmetrical set of rotating voltage and current vectors and to control the static switches to instantaneously reverse the rotation of the vectors.
A further object of the present invention is to provide an N-stage inverter system for a harmonic neutralized static power generator in which an N-stage phase shifter arrangement is so controlled as to provide for instantaneous reversal of the outputted waveform.